1. Field of the Invention
This invention relates to powder metallurgically formed steels, and particularly to such steels having enhanced corrosion resistance, and more particularly to h-BN (hexagonal boron nitride) additions to such steels to accomplish enhanced corrosion resistance as well as increased hardness, tensile strength, free machining properties, tightness and surface density. In particular, stainless steels of both austenitic and ferritic type are especially suitable for being produced using a method according to the invention. Powder metallurgy will be referred to as P/M henceforth.
2. Description of the Prior Art
A sintered stainless steel is known where an addition of boron is made to improve the corrosion resistance and the mechanical properties, for example from U.S. Pat. No. 4,032,336 (Reen) which is hereby incorporated as reference. Improved corrosion resistance and improved mechanical properties are due to increase in density. The boron forms a liquid phase during sintering, depleting chromium and molybdenum from the steel powder. The steel powder therefore contains sufficient amount of Cr and Mo to offset this depletion which results in the sintered non-melted parts of the product being within the required composition for a specific austenitic stainless steel. Boron is added to the base material to obtain a pre-alloyed metallic powder which (according to the ASTM handbook Volume 7 p.9) is a metallic powder composed of two or more elements that are alloyed during the powder manufacturing process, and in which the particles are of the same nominal composition throughout.
The raw material thus contains an elevated amount of Cr and Mo, which adds to the cost of the raw material.
According to JP 01-129903 (Wataru), of which the JAPIO English abstract is hereby incorporated by reference, hexagonal boron nitride (h-BN) is mixed with a metallic powder (preferably an iron alloy containing Co, Ni, Cr, etc.). The purpose of adding h-BN to the metal powder is to enable compaction without using an organic lubricating agent, thus utilizing h-BN as a lubricating agent.